Process for the secondary recovery of petroleum

ABSTRACT

Crude oil is recovered from a subterranean oil bearing formation by injecting an initial slug of an aqueous flooding medium containing a dissolved polymeric agent into the formation to displace the oil toward at least one output well communicating with the formation and then following the initial slug with a secondary aqueous slug having a dissolved polymer concentration of less than about one-third that of the initial slug but not in excess of 100 p.p.m.

united States Patent Jennings 1 1 Aug. 29, 1972 [54] PROCESS FOR THESECONDARY 3,370,649 2/ 1968 Wogemuth ..166/274 RECOVERY OF PETROLEUM3,406,754 10/1968 Gogarty ..166/273 3,434,542 3/ 1969 Dotson et a1...166/273 [72] Invenmr' Jenmngs Walnut Creek 3,467,187 9/1969 Gogarty etal. ..166/273 Assigneer h Dow Chemiw] p y Primary Examiner-Stephen J.Novosad Mldland, Mlch- Attorney-Griswold & Burdick, Herbert D. Knudsen22 Filed: May 7, 1970 and Rehberg [21] Appl. No.: 35,486 57 ABSTRACTCrude oil is recovered from a subterranean oil bearing [52] US. Cl..166/274, 166/273 formation by injecting an initial Slug f an aqueous[51] Ill-ll. Cl. ..E21b 43/22 flooding medium containing a dissolvedpolymeric [58] Flew M Search "166/273, 305 252 agent into the formationto displace the oil toward at least one output well communicating withthe forma- [56] References Cited tion and then following the initialslug with a seconda- UNITED STATES PATENTS ry aqueous slug having adissolved polymer concentration of less than about one-third that of theinitial slug 3,018,826 l/ 1962 Sand1ford ..166/273 X but not in excessof 100 ppm 2,827,964 3/1958 Sandiford ..166/274 3,039,529 6/1962McKennon ..166/275 7 Claims, 1 Drawing Figure 6 I I I I l 1. I 5 1 l 1 1I l 5 O l R j -2 s 1 K 1 25 .m TE) 3 1 L .L iii. 1 Q l 0 1 Q 2 1 B/v'n eBrine l 0 5O loo /50 200 250 300 350 Cu/nu/O/fvz? vo/ume m/ PROCESS FORTHE SECONDARY RECOVERY OF PETROLEUM 1 BACKGROUND OF THE INVENTION Thesecondary recovery of oil is well known. In some secondary recoverymethods water or brine solution is forced into an oil bearing formationto push the oil towards at least one output well. When only water orbrine solution is employed without the addition of polymeric agents, theefficiency of the recovery is severely hampered in many cases becausethe flooding agents by-pass much of the oil that could be recovered. Asa result of this and other deleterious aspects of using water or brinealone, polymeric agents have been incorporated into the pushing fluid toincrease the viscosity of the fluid and decrease the mobility of thesolution through the formation. As a result, the efficiency of the oilrecovery has been increased. It has been proposed that fairlyconcentrated solutions of viscosity enhancing polymers be used as oildisplacing fluids. These solutions may be technically efficient in therecovery of the oil but are economically unacceptable. Polymersespecially suited for such applications are described by Pye in U. S.Pat. No. 3,282,337.

Gogarty in US. Pat. No. 3,406,754 teaches an attempt to make thesecondary oil recovery more economically feasible by graduallyincreasing the mobility of the flooding medium from a low point at theoil interface to a high mobility of the primary drive material. Toaccomplish this, Gogarty uses an initial soluble oil slug, awater-extemal emulsion and then a thickened fluid drive material havinga gradually increasing mobility to a mobility of essentially pure water.

SUMMARY OF THE INVENTION In the process of the present invention, crudeoil is recovered from a subterranean oil bearing formation by injectinginto the formation an initial slug of an aqueous flooding mediumcontaining a water-soluble, substantially linear organic polymercharacterized by a resistance property, hereinafter defined, of at least1.5, and then injecting a secondary aqueous slug having a dissolvedpolymer concentration up to about one-third that of the initial slug butnot in excess of 100 ppm. By the use of this technique, the beneficialeffect of the initial relatively concentrated slug is economicallyretained.

The essential and novel feature of the invention involves following theprincipal polymer bank in a secondary oil recovery process with asolution of the polymer which is much more dilute than that of theprincipal polymer bank while maintaining at least some dissolved polymerin the flooding medium. The surprising feature of the invention is thehigh retention of the reduced mobility characteristic indicative of theprincipal bank even when the polymer concentration is substantially lessthan a third the concentration of the initial bank. Not only is theefficiency retained, but the use of smaller quantities of polymer lowersthe effective cost of secondary oil recovery.

The FIGURE shows a representative plot of the mobility factor of polymersolution in a typical oil field core for varying concentrations ofpolymer put thro gh the same core as will be set forth in detailhereinafter.

The displacement of oil in secondary oil recovery is affected by asubstantial number of variables, not all of which are known. Some of theknown variables, however, include: those relating to the oil bearingformation, such as the heterogeneity of the formation, the geometricalarrangement of the injection and the withdrawal points, the permeabilityof the formation and the mobility of the oil; and those relating to theflooding medium, such as the nature and viscosity of the floodingsolutions and the mobility of the solution in the formation.

The most important combination of these variables is the mobility of theoil in the formation compared to the mobility of the flooding solution.If the mobility of the oil is low and the mobility of the floodingsolution is relatively high, the efficiency of the oil displacement isquite low because the higher mobility of the flood causes the solutioninjected to pass by some of the less mobile oil and eventually fingerthrough to the production well.

This same phenomenon exemplifies the possible problems associated withthe injection of two polymer banks where the second bank has asubstantially lower polymer concentration. It would be expected that thesubsequent dilute bank would tend to finger through the preceding bankand the less mobile oil to decrease the efficiency of the recovery.

It would also be projected that the problem of fingering through wouldbe accentuated by subterranean formations of heterogeneous permeability.Indeed, this is the major drawback of aqueous flooding. In suchsecondary oil recovery water fingers through the zones of highpermeability and blocks recovery of oil from those zones of lowerpermeability. Technically, this problem has been solved by the additionof relatively high polymer concentrations which substantially decreasethe mobility of the slug through the zones of high permeability in anattempt to equate the mobility of the fluid through the sectors of theheterogeneous formation. Sustained use of such concentrations, however,is economically unacceptable.

Surprisingly, it has now been found that fingering by a significantlyless concentrated polymer solution is minimal or nonexistent, and thebeneficial efiect of relatively high polymer concentrations issubstantially retained during flooding with subsequent dilute banks evenin formations of heterogeneous permeability.

The important quantative measurement of the comparative mobilities inthe invention is the mobility factor. This mobility factor is expressedas the resistance to flow of the solution tested (usually the polymersolution) relative to the resistance to passage of the brine or aqueoussolution through a sample of the formation having residual oilsaturation before any treatment with polymer. This mobility factor isdefined as:

where AP= pressure drop K eflective permeability p. solution viscosity MK/p. mobility S solution tested, usually the polymer solution B water orbrine A mobility factor of 3 simply indicates that the solution testedis three times as hard to push through the sample as the water or brine.The comparative mobility factors of various polymer solutions on thesample core can be correlated to the recovery of oil from the formation.

In a practical application, the interreaction between the proposedreservoir formation, the oil, the brine or water to be used as theflooding medium, the nature of the polymer, the concentration at whichthe polymer is initially employed, the concentration of any subsequentpolymer-containing brines and the various other factors involved insecondary oil recovery are so complex that a prediction of the effectscannot be accurately made without some prior approximation. By usingcores from the formation and evaluating the comparative mobilityfactors, the proper application of the invention may be convenientlyobtained. Although this information may be applied to approximate theactual conditions of the secondary oil recovery, the experimentalanalogy cannot be completely accurate. The inaccuracies, however, resultmainly from the unavailability of samples which are indicative of thetotal formation.

The invention may be applied to a specific formation by simply followingthe steps outlined below:

a. Representative samples of the formation and fluids involved areobtained;

b. Small right cylinders of the formation measuring 1 inch in diameterby 1 inch high are prepared and placed in a test apparatus designed toflow liquids through the specimen measuring the pressures and flows; ifthe formation is not mechanically strong enough to support a cylinder,special techniques may be used in preparing the test specimen;

c. The sample is flooded with brine from the formation and then withoil, preferably the oil of the reservoir. This oil is then displaced ascompletely as possible by water or brine equivalent to that proposed forthe injection to give a sample having residual oil saturation. At allstages, the pressures and flow rates are monitored to permit thecalculation of the mobility factors of the different fluids in thespecimen;

d. A relatively concentrated solution of a polymer having a resistanceproperty of 1.5 in the proposed injection water is passed through thecore. The polymer flooding solutions are those usually employed inconventional applications; for example, a concentration of partiallyhydrolyzed polyacrylamide between 200 and 1,000 ppm. would be typical.The mobility of this solution is compared to that of the injection waterrun through the specimen before the injection of polymer to give themobility factor of the concentrated slug;

e. To determine whether the invention is applicable to the formation, apolymer-free injection fluid is then introduced in quantities sufficientto reach a stable flow condition as indicated by the invariance of thepressure and flow rate as several pore volumes of fluid are passedthrough the core. After steady state conditions have been attained, themobility factor associated with the polymer-free solution is compared tothat of the initial polymer bank. If the polymer-free solution shows amobility factor at equilibrium substantially below that of the initialpolymer bank, a secondary bank with a reduced polymer concentrationwould be considered to be of possible benefit;

f. The initial relatively concentrated polymer solution is againintroduced into the specimen until stable conditions exist. Thisreintroduction of the initial polymer bank is required because thebenefits of a lower concentration of polymer in the aqueous solution canonly be realized if the less concentrated solution is introducedimmediately after the concentrated slug;

g. Solutions containing lower concentrations of the polymer should thenbe tested until the levels are not capable of sustaining a sufficientlyhigh mobility factor to be of interest. Economics will usually requirethat this concentration of the subsequent polymer flow be less thanone-third that of the initial polymer bank, with concentrations ofone-fourth to one-tenth generally being preferred. Of course, mosteconomical would be the lowest concentration that would givesatisfactory retention of the desirable mobility factor; and

h. After the examination has given the desired results, the core may beflushed with water or brine to equilibrium and a different concentrationof the initial polymer bank may be tested with subsequent evaluations ofthe effectiveness of lower polymer concentrations. Also, other polymersmay be tested to determine their effectiveness in the invention.

The test procedures as outlined above provide an effective andconvenient method of determining the concentration of the initial slugand the concentration of I subsequent slugs in secondary oil recovery.Since the formation may be substantially heterogeneous, it is desirableto make a number of tests on various cores of the formation to giveexperiments which can be readily correlated to the actual situation.With representative examples, the procedures outlined provide a goodanalogy of the actual oil recovery from a formation by secondarytechniques.

The polymers employed in the aqueous solutions can be any of thewater-soluble organic polymers or mixture thereof which are sometimesreferred to as hydro philic polymeric colloids. These polymers aregenerally characterized by substantial linearity and high molecularweight. As noted above, these polymers must have a resistance propertyof at least 1.5. Such resistance property is defined by Pye in US. Pat.No. 3,282,377 according to the formula:

Resistance Property R In the formula 1 is the viscosity of 0.05 percentby weight solution of the water-soluble polymer in a 3 percent by weightaqueous sodium chloride solution, such viscosity being measured with anOstwald viscosimeter, i.e., determined by means of measuring flow ratesthrough a capillary at 25 C. Q, is the flow rate in milliliters perminute, under a given pressure, of a 0.05 percent by weight solution ofthe polymer in a 3 percent by weight aqueous sodium chloride solution,lengthwise through a right cylindrical core of Berea sandstone, one inchin diameter and one inch long, the core having been previously saturatedwith the 3 percent sodium chloride solution. The sandstone core used inthis measurement has, when dry, a permeability to air within the rangeof to 500 millidarcys. The pressure at which the flow rate is measuredis that pressure required to flow a solution of 3 percent by weightsodium chloride in water through the sandstone core at a rate of 30milliliters per minute.

These polymers are dispersible in water to provide a visuallyhomogeneous system during flooding operations. Various water-solublepolymers that are suitable for the present invention are described byPye, with polymers having ionizable groups such as sulfate, sulfonate,carboxylate, carboxamide, amino and ammonium groups being operable.Especially pertinent high molecular weight polymers are obtained byaddition or condensation polymerization of the appropriate monomers. Thepreparation of these polymers is found among the addition polymersobtained by ethylenic polymerization such as those described in Hedricket al. in US. Pat. No. 2,625,529; Aimone et al. in U. S. Pat. No.2,740,522; and Booth et al. in U. S. Pat. No. 2,729,557. A variety ofpolysaccharide derivatives are described in Gloor in U. S. Pat. No.2,728,725. Polyurethanes or chain extended polyols will be found inHonea et al. in U. S. Pat. No. 3,054,778 and a number of polycarbarnatesand lactams are found in l-libbard et al. in U. S. Pat. No. 3,044,992;Wales et al. in U. S. Pat. No. 2,946,772; Vitales in U. S. Pat. No.2,874,124; and Fong et al. in U. S. Pat. No. 3,000,830. Further polymersmay be prepared as shown by Davidson and Sittig in Water-Soluble Resins,Reinhold Publishing Company, New York, 1962. Polyacrylamides andparticularly polyacrylamides having from about 15 to 30 percent of theircarboxamide groups hydrolyzed to carboxyls are the polymers of specialinterest in the present invention.

The amount of the initial polymer bank introduced into the formation mayvary widely. Since the polymers and formations differ substantially fromapplication to application, the amount of the initial polymer bankcannot be precisely defined. The amount, however, may be approximated byreference to the preliminary mobility testing as described above usingthe core samples and also from a study of core logs and other tests ofthe formation under consideration. These tests will generally give anindication of how much of the initial relatively concentrated polymersolution need be passed into the formation to give the necessary basefor the addition of dilute polymer solutions. In practice it isgenerally found that adequate mobility control is accomplished with aninitial slug constituting from about to about 50 percent, preferablyfrom about 20 to about 30 percent, of the pore volume of the formationbeing treated. in any case, such initial slug will be designed to besufficient to assure that the subsequent dilute polymer solutions willretain the desired mobility factor.

The concentration of the water-soluble polymer in the aqueous floodingmedium of the initial bank may also vary widely again depending upon thepolymer employed, the oil displaced and the particular formation. Theresultant polymer solution usudly has a viscosity of at least one-halfof 1 percent of the crude oil, with polymer solutions having a viscosityof at least 5 percent that of the crude oil being preferred. The highestconcentration is limited only by practical considerations of workabilityand economy.

The secondary bank of polymer solution or dilute polymer solution has aconcentration of up to about one-third that of the initial polymer bankbut not in excess of 100 ppm. Although different polymers could be addedto the secondary bank of the flooding, use of the same polymer in bothslugs is much preferred.

Thus, any of the polymers or mixtures thereof that may be used in theinitial relatively concentrated bank can be used in the secondary bankalthough they are present in a lower concentration. The concentration ofthe secondary solution must be less than one-third of the concentrationof the original bank, or if different polymers are employed, theconcentration should produce a similar mobility factor as a secondarybank of the same polymer. Solutions of the secondary polymer havingone-fourth to one-tenth the concentration of the initial polymer bankand a mobility factor at least twice that of the subsequent flood withwater or brine after polymer treatment are preferred. Also preferred isa subsequent slug having a polymer concentration of at least about 10ppm.

The optimum concentrations which give the highest mobility factor withthe least amount of polymer may be determined or at least approximatedfrom the core models. in such determination, the initial slug ofconcentrated polymer solution is followed by a dilute secondary slug inthe same proportions and concentrations proposed for the oil recoveryoperation. The slugs which gave the highest sustained mobility factorwith the least amount of polymer is the optimum solution if theformation conforms to the core sample.

From the discussion above, it can be readily seen that differenttechniques may be employed to accomplish the same results of theinvention. For example, the polymer concentration may be abruptlyreduced more than once. Also covered by the present invention is therapid gradient concentration reduction of the polymer solution asopposed to the gradual reduction in the concentration of polymersolution shown in the art.

SPECIFIC EMBODIMENTS E "I?" l A Berea sandstone specimen in the form ofa 1 inch diameter by 1 inch high right cylinder having an airpermeability of 250 millidarcies was placed in a test apparatus whichconsisted of a hollow cylinder which fit tightly around the specimen toprevent the flowing of any liquid between the wall of the hollowcylinder and the specimen but at the same time providing an excesshollow space below and above the test specimen. The sample had a porevolume of 2.25 ml. The specimen was flooded with a brine containing 4percent sodium chloride and 0.3 percent calcium chloride dihydrate. Thespecimen was then saturated with a refined oil having a viscosity of 6.5cps., and the oil was displaced with a large volume of brine. Thissequence of fluids resulted in the establishment of an irreduciblesaturation of oil in the porous spaces of the rock. The flow rate of thebrine was then determined at a measured pressure.

Solutions of a partially hydrolyzed, substantially linear, highmolecular weight polyacrylamide sold under the trade name Pusher P700were prepared. The initial concentrated bank was a 250 ppm. solutionhaving a viscosity of 1.35 cps. The treatment of the sample with thisinitial slug and subsequent slugs having a lower concentration are shownin the FIGURE.

it will be noted that a 5 fold reduction in the concentration, overpercent of the original mobility factor was retained and at a 10 folddilution, the mobility factor was about 50% that of the originalconcentrated slug.

EXAMPLE 2 After completion of primary oil production from a formation ofwhich the core of Example 1 constitutes a representative sample, the oilfield in integrated under unitary management and secondary recovery isinstituted by pumping brine, having substantially the same compositionas-the connate water in the formation, into a plurality of selectedinjection wells penetrating said formation and producing fluids from aplurality of production wells properly spaced from the injection wellsand likewise penetrating the formation. Shortly after initiation ofsecondary recovery, it is found by the use of tracers that injectedbrine is appearing in production well fluids and when the ratio of waterto oil in the produced fluids rises to 3 to 1, it is decided to initiatepolymer flooding. After providing suitable tanks, pumps and injectionvalves, a solution is prepared containing 0.5 percent by weight ofhydrolyzed polyacrylamide in the brine, the polyacrylamide beingsubstantially linear and of high molecular weight, being characterizedby a viscosity of about 25 centipoises for a 0.4 percent by weightsolution thereof in 4 percent sodium chloride brine at a temperature of25 C., and having about 20 percent of its carboxamide groups hydrolyzedto carboxylate groups in the sodium salt form. The resulting solution isintroduced through a proportioning pump into the main brine supplyflowing to the injection wells in amount sufficient to provide 300 partsby weight of the polymer per million parts of medium injected into theoil-bearing formation. Injection of polymer solution is continued untila volume corresponding to 25 percent of the pore volume of the formationhas been injected. During this period, the injection well pressuresincrease and the water-cut of the fluids in the production wells isstabilized. The polymer dosage is thereupon decreased to provide aflooding medium containing 25 parts of polymer per million parts ofmedium and flooding with this latter medium is continued. It is foundthat injection pressures are maintained at a desirable high levelwithout evidence of premature break-through of the flooding medium.

In the same manner as shown by the examples above, the techniquesapplied to the core samples may be applied to secondary oil recovery byintroducing into an oil bearing formation the hydrolyzed polyacrylamideor other water-soluble, high molecular weight polymer in a water floodat an initial relatively high concentration and then substantiallyreducing the concentration with a subsequent flood while retaining ahigh proportion of mobility control properties to improve the efficiencyof oil displacement.

lclaim:

1. In the process for the secondary recovery of oil from a subterraneanformation by injecting into an oil bearing formation an aqueous floodingmedium containing a dissolved polymeric agent to displace the oil towardone or more output wells communicating with the formation, theimprovement comprising injecting into the formation an initial aqueousslug having a dissolved polymeric agent with a resistance property of atleast about 1.5, and then injecting a secondary slug having a dissolvedpolymer concentration of up to about one-third that of the initial slugbut between about 10 and p.p.m. for the use of the same polymer or forthe use of npthe r polymer, a solu ion ha produces a similar mo 1 rtyactor as a secon ary slug of the same polymer.

2. The process of claim 1 wherein the secondary slug contains the samepolymer as the first slug.

3. The process of claim 1 wherein the viscosity of the initial slug isgreater than about one-half of 1 percent of the viscosity of the oil tobe displaced.

4. The process of claim 1 wherein the concentration of the secondaryslug is one-fourth to one-tenth that of the initial slug.

5. The process of claim 1 wherein the mobility factor of the secondarypolymer bank is at least twice the mobility factor of the brine or waterafter polymer treatment.

6. The process of claim 1 wherein the dissolved polymer is apolyacrylamide.

7. The process of claim 6 wherein the polyacrylamide is partiallyhydrolyzed.

2. The process of claim 1 wherein the secondary slug contains the samepolymer as the first slug.
 3. The process of claim 1 wherein theviscosity of the initial slug is greater than about one-half of 1percent of the viscosity of the oil to be displaced.
 4. The process ofclaim 1 wherein the concentration of the secondary slug is one-fourth toone-tenth that of the initial slug.
 5. The process of claim 1 whereinthe mobility factor of the secondary polymer bank is at least twice themobility factor of the brine or water after polymer treatment.
 6. Theprocess of claim 1 wherein the dissolved polymer is a polyacrylamide. 7.The process of claim 6 wherein the polyacrylamide is partiallyhydrolyzed.